How do I get my app to use the Reader when both Reader and Acrobat Standard are installed?

My 3rd party app doesn't respect the default setting for PDFs. I have both Adobe reader and Adobe Acrobat standard installed.  Adobe Reader is set as the default PDF reader in Windows XP but my 3rd party app opens the PDFs with Acrobat Standard instead.  Only when I uninstall Acrobat Standard does it then open PDFs with Reader. Is there any information I can send to the 3rd party support people who are telling me this isn't there problem? They say they only use what the OS provides.